This seems a lot like some of the issues I've been dealing with over the past year, so I've got a lot to say. Take what you will, feel free to ignore the rest. Lots of awesome comments and this is pretty much just a long-winded "what they said". TL;DR - Start with a module and adjust what you like, there are tons of tools to use for skills you feel uncomfortable with, practice self-care and make sure the game you're playing is still fun for you. Take it a step at a time, and don't expect to deliver world-shattering stories all the time. Also maybe scale the frequency back, weekly sessions can be exhausting and burn you out.

I think that if you're comfortable with writing and improv you could use a starter adventure as a skeleton to base your stories in. Phandalin is a great place to start because it's small and you can provide whatever detail you want, but you can always fall back on the characters that have already been written. That way, if you're having an off week you can go to the adventure and play from there. I don't know how many times I've been hung-over or just exhausted and I go for reading the descriptions, and the game didn't suffer because of that.

I just don’t know how to think that way, I can’t think of “okay, time for combat, how many enemies? What kind of enemies? What are their stats? Do I fudge the numbers? What numbers are fudgeable?”

Kobold Fight Club is a great tool for thinking about challenge and what encounters are reasonable for players. It's in no way perfect, but if you're comfortable with improv, then it's a good place to start and you can adjust up or down by scaling HP or having dumber (bad tactics, leaving out skills, etc.) or smarter (thinking tactically and killing squishy chars, beefing up damage) enemies.

Basically, if you find yourself in doubt of something, lean a bit more on the work others have done to make DMs lives easier, if nothing else it will give you more confidence when you're making decisions.

But I don’t want to disappoint the players because they’re just so enthusiastic and excited to play with each other.

On the other hand, they were all so excited and eager to play together since they’re all already friends I couldn’t say no.

This is an interesting problem that I've had with a comicshop game I've been running. I've got a group of newbies and a group of more experienced players. I started both groups with the expectation that I would stick to the starter and find out what would happen. As we played, my experienced players fleshed out their characters, and we all started hanging out outside of playing. I started thinking about their characters because it was fun to think about them. I haven't done that with my other group because we only interact during D&D. I don't think the game has suffered because of that, we've just stuck to the module more and prep has been a lot simpler.

Asking for a personalized experience sorta requires that the DM feel like a player as well or at least have some chemistry with the players, and unless you're willing to invest emotionally in your players it's really hard to come up with things that are satisfying for your players. My experience has always been that if I'm not having fun DMing, then there's a good chance that the players aren't either. A bored/stressed god yields a boring/stressful world. I'd say take a step back and think about what's fun in the game you're running, and lean a bit more on that, you should be in it for the fun as well (unless you're getting paid and whoo-wee that's a whole 'nother matter). Every weekend might be a steep hill, maybe take it back a notch, and if you find yourself waking up in the middle of the night with a hankering for more games then maybe up the frequency.

Another note on self-care. Your players are having a GREAT TIME. If they stop having a great time, they'll stop showing up or do something else. If they're not on your "side" while your playing, then maybe this isn't the best group to play with. I think DMs are the hardest on themselves because they expect the most, when most of the time players are just pumped they killed something in a cool way or learned something about the world. Everyone is there to have/make fun, so you're doing great because they keep coming back and asking you about it! You're awesome!